Abstract: A combination of piston rings assembled to a piston includes a first compression ring, a second compression ring, a third compression ring and an oil ring. When an axial width of the first compression ring is h1(1), an axial width of the second compression ring is h1(2), an axial width of the third compression ring is h1(3) and an axial width of the oil ring is h1(4), h1(1)?h1(2) and h1(1)?h1(3), and when h1(TOTAL)=h1(1)+h1(2)+h1(3)+h1(4), h1(TOTAL)?3.1 mm.

Abstract: A combination of piston rings assembled to a piston includes a first compression ring, a second compression ring, a third compression ring and an oil ring. When an axial width of the first compression ring is h1(1), an axial width of the second compression ring is h1(2), an axial width of the third compression ring is h1(3) and an axial width of the oil ring is h1(4), h1(1) ? h1(2) and h1(1) ? h1 (3), and when h1 (TOTAL) = h1 (1) + h1(2) + h1(3) + h1 (4), h1 (TOTAL) ? 3.9 mm.

Abstract: A wet-type plate-making machine is disclosed in which the amount of a developer in a developing tank and the toner concentration of the developer are maintained constant. In accordance with the detection values of two liquid level detection switches mounted in the developing tank, the developer is supplied from a developer refill pump thereby to maintain a constant amount of developer. The toner concentration of the developer is maintained constant by supplying the toner from a toner refill pump when the detection value of a concentration detection unit of light-transmission type is low, and by supplying the developer from a developer refill pump when the detection value of the concentration detection unit is high. In the case where the developer concentration or the toner concentration fails to change in this control operation, an alarm signal is outputted indicating that the developer bottle or the toner bottle, as the case may be, is empty.